... TECHNICAL REPORT WA/91/4 Applied geological mapping in the Wrexham area: geology and land- use planning B A Hains .. .- BRITISH GEOLOGICAL SURVEY TECHNICAL REPORT WA/91/4 Onshore Geology Series Applied geological mapping in the Wrexham area: geology and land-use planning B A Hains Contributors M G Culshaw Engineering geology R A Monkhouse Hydrogeology Cover illustration The A483 from Chester to Wrexham (Gresford bypass)has been built through the northern part of the gorge of the River Alyn, partly through on soft alluvial sediments, and partly on waste from the now-disused Gresford Colliery. This study was commissioned by the Department of the Environ- ment, but the views expressed in it are not necessarily those of the Department Maps and diagrams in thisbook use topography basedon Ordnance Survey mapping Geographical index UK, Wales, Clwyd Subject index Land-use planning, thematic maps, resources, aggregate, mining, engineering geology, ground stability, hydrogeology Bibliographic reference Hains, B A. 1991.Applied geological mapping in the Wrex- ham area: geology and land-use planning. British Geological Survey Technical Report WAl9114. 0 NERC copyright 1991 Keyworth,Nottingham British Geological Survey1991 BRITISH GEOLOGICAL SURVEY The full range of Survey publications is available through the Keyworth, Nottingham NG12 5GG Sales Desks at Keyworth and at Murchison House, Edinburgh, e Plumtree (06077) 6111 Telex378173 BGSKEY G and in the BGS London Information Office in the Natural Fax 06077-6602 History Museum Earth Galleries. 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St Just, 30 Pennsylvania Road, Exeter EX4 6BX The British Geological Survey is a component body of the Exeter(0392) 78312 Fax 0392437505 Natural Environment Research Council. e Bryn Eithyn Hall, Llanfarian, Aberystwyth, Dyfed SY23 4BY e Aberystwyth(0970) 611038 Fax 0970-624822 Windsor Court, Windsor Terrace, Newcastle upon Tyne NE2 4HB 091-2817088 Fax 091-2819016 Geological Survey of Northern Ireland, 20 College Gardens, Belfast BT9 6BS e Belfast(0232) 666595 Fax 0232-662835 Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX10 8BB B Wallingford(0491) 38800 Telex 849365 HYDROL G Fax 0491-25338 Parent Body Natural Environment Research Council Polaris House, North Star Avenue, Swindon, Wiltshire SN2 1EU e Swindon(0793) 411500 Telex 444293 ENVRE G Fax 0793-411501 Printed in England for the British Geological Survey by Baker Brothers Litho Limited, Pontefract, West Yorkshire PREFACE This account describes the applied geology of the Wrexham area, encompassing 215 sq km. It covers theOrdnance Survey 1:lO 000 Sheets SJ 24 NE; SJ 25 NE, SE; SJ 34 NW,NE; SJ 35 NW, SW, SE; and partof SJ 35 NE. The northern partof the district was first geologically surveyed at the 1:10 560 scale between 1879 and 1885; the remaining area was completed, and the northern part partially revised, from1910 to 1913. Publications on economic aspects of the district included a memoir on the metalliferous mining and mineral resourcesof the region (Smith,1921) and two reportsgiving assessments of the sand and gravel reserves (Dunkley,1981; Ball, 1982). The presentstudy was commissioned by the Departmentof the Environment onbehalf of the Welsh Office. Funding was provided jointly by the Department of the Environment and the British Geological Survey.Its objectives wereto produce updatedgeological maps at1:lO 000 scale, and, together with other available surface and subsurface data, to present thegeological information relevant to land-use planning, development and redevelopment in the form of a set of maps at 1:25 000 scale with accompanying reports. A major aim of the study was to develop computer techniques so that the geological and thematic mapscould all be computer generated on demand and in colour (Loudon et al., 1991) if required. Particular emphasis has been placed on drift deposits, undergroundworkings and mineral resources. Synthesesof the engineering propertiesof the bedrock and drift (superficial) deposits of the area were undertaken (Waine et al., 1990). Limited testing of Coal Measure mudstones to determine their swelling characteristics has been carried out (Entwisle, 1989). A summary of the hydrogeology has been providedby Mr RA Monk- house. Detailed resurvey at 1: 10000 scale, by Drs J R Davies and D Wilson, was limited to the Hope Mountain area;a desk studyof the remainderof the area,including partial photogrammetric interpretation, andlimited field checking was carried out by Drs R Addison (Leeswood Coalfield), B A Hains and TI Williamson (Sheet SJ 35 NE). DrN J Riley provided palaeontological support for the Hope Mountain area. DDr J Evans provided geophysical information pertaining to geolo- gical structures in the eastern partof the area. Datacollection and computer coding of borehole logs was carried out by Mr J A Thorburn.The Project Leader was Dr BA Hains. Dr R A B Bazley, Regional Geologist for Wales, was the Nominated Officer for the British Geological Survey, and Mr D B Courtier, Welsh Office, was the Nominated Officer for the Secretary of State. The ready cooperation of landowners and tenants, quarry companies, local authorities, govern- mental bodies andother holders ofdata during this surveyis gratefully acknowledged.In particular we thank Clwyd County Council, Alyn and Deeside District Council, Wrexham Maelor Borough Council, British Coal, the Welsh Development Agency, Astbury Quarries Ltd, Dennis Ruabon Ltd, Alfred McAlpine Quarry Products Ltd, Tarmac Roadstone Ltd (North West) and Welsh Aggregates Ltd. We also acknowledge many geological and geotechnical consultants. V CONTENTS ExecutiveSummary 1 Annex B Data sources 81 Introduction4 C Annex 84Geotechnical database Objectives4 The use of this reportand limitationsits 4 Annex database 88 Presentation of results 6 AnnexE Glossary 89 Geographical and planning background 7 Geographical background 7 Planning background 11 FIGURES Geological background (Thematic 1 Sketch-map of the study area showing its Maps 1-4) 16 regionalsetting 8 Introduction16 2 Sketch-map showing the component Solid geology 18 1: 10 000sheets of the study area and the Drift geology 20 outlines of the earlier County Sheet Grid 9 Rockhead elevation 24 3 Topography and drainage of the study Drift thickness 27 area 10 Mining and mineral resources (Thematic 4Planning factors 13 Maps 5-7) 29 5 Agriculturalland classification 15 Mining activities - CoaVMetalliferous 29 6Generalised geological sequence 17 Bedrock resources - except 7 Solid geology 19 CoaVMetalliferous 37 8 Drift geology 21 Sand and gravel resources 41 9Rockhead elevation 25 Slope steepness 45 11 Drift thickness 28 12 Mining activities 30 Hydrogeology (Figure 18) 52 13 Correlation of the Bettisfield Formation (Productive Coal Measures) 32 Geological factors for considerationin land-use 14 Bedrock resources 36 planning (Thematic Map 10, Figure 19) 56 15 Sand and gravel resources 42 Mineral resource potential 57 Physical and chemical constraints to 16 Engineering geology -46solid development61 17 Engineering geology -48drift 18 Hydrogeology 52 References 66 19 Mineral resource potential 58 20 Geological factors for consideration in Annex A Summary of geology 70 land-use planning 59 vi TABLES ‘Correlation’ of Coal Seam nomenclature for theLeeswood and Wrexham areasof the North Wales Coalfield 34 Summary of mechanical and chemical property data from limestones 38 Engineering geology units- Solid geology 47 Engineering geology units- Drift (superficial) deposits 49 Slope steepness classes and examplesof land use limitations 51 Licensed abstraction from shafts, bore holes and springs 53 Landfill waste disposal sites54 Chemical analyses of water sources 55 vii EXECUTIVE SUMMARY Introduction The study area covers the northern part of the ‘Denbighshire’ Coalfield and its environs. It is centred on Wrexham, thelargest town in North Wales. This area has beenundergoing a transfor- mation with the decline of coal mining and associated heavy industries and their replacementby an expanding service sector and new light and high-technology industries. Two other significant parts of the local economy, agriculture and the mineral extraction industry, continue toplay an important de. The decline of the coal industry hasleft a legacy of old workings, abandoned shafts and adits, and spoil heaps. Additionally there are many disused and extant workingsfor sand and gravel, limestone and brick-clay. This history of exploitation of mineral resources